1. Field of the Invention
The present invention relates to an AC bias control circuit and, more particularly, is directed to an AC bias control circuit for superimposing an AC bias current onto a recording head in a magnetic recording apparatus.
2. Description of the Related Art
When a signal is recorded on a magnetic tape through a recording head, a predetermined amplitude of AC bias current is superimposed onto a recording signal current in order to eliminate the distortion of the signal to be recorded to thereby widen the dynamic range of the recorded signal.
FIG. 1 shows a relationship with respect to a low-frequency band signal between the quantity of AC bias applied to the recording head and the maximum signal level which can be recorded on a magnetic tape . As is apparent from FIG. 1, the signal level becomes the maximum when the AC bias current is in the vicinity of +6 dB.
FIG. 2 shows a relationship with respect to a high-frequency band signal between the quantity of AC bias applied to the recording head and the maximum signal level which can be recorded on a magnetic tape. As is apparent from FIG. 2, the signal level becomes the maximum and minimum when the AC bias current is in the vicinity of -6 dB and +6 dB, respectively.
There is, therefore, no optimum bias current common to both the low- and high-frequency band signals, and conditions for obtaining an optimum bias current common to both the low- and high-frequency band signals are contradictory to each other. Heretofore, therefore, a bias current at an intermediate position between the optimum points for both the low- and high-frequency band signals was selected as compromise of those contradictory conditions.
The above-mentioned maximum signal level depends on also a signal to be recorded. That is, when the recording signal is a broad band signal over from a low-frequency band to a high-frequency band, a high-frequency band signal has a biasing effect on a low-frequency band signal so that the quantity of effective bias becomes large.
Therefore, in this case, there arises a problem that a set bias point was changed by a recording signal, so that the bias point becomes deeper than the optimum value. In addition, since a high-frequency band signal has a biasing effect in itself, there arises a problem that the biasing point becomes deeper than the optimum value even with respect to the high-frequency band signal.
Such a circuit arrangement as shown in FIG. 3 has been proposed as a circuit for overcoming the aforesaid problems. In FIG. 3, the circuit includes a recording head 1 and a low-pass filter 4 which is constituted by a resistor 2 and a capacitor 3.
The circuit further includes a peak detector 5, and a differential amplifier 6. The differential amplifier 6 has an input 7 connected to the output of the peak detector 5 and another input 8 connected to a reference voltage source 9 so that a differential signal between input signals applied to the inputs 7 and 8 is developed at its output 11.
The output of an oscillator 12 is applied to an input of a voltage control amplifier 10 and the output of the voltage control amplifier 10 is applied to one input of an adder 14. The output of the differential amplifier 6 is applied to a gain control input 11 of the voltage control amplifier 10.
A recording signal input terminal 13 is connected to the other input of the adder 14. The output of the adder 14 is applied to the recording head 1. The output of the oscillator 12 is applied also to an erasing head 15. A reference numeral 16 represents a magnetic tape.
The operation of the circuit of FIG. 3 will be described. When recording a signal onto the magnetic tape 16, the tape 16 is transported from left to right in FIG. 3. If there are signals recorded on the tape 16, those signals are erased by the erasing head 15. At that time, an oscillation output B is applied from the oscillator 12 to the easing head 15.
A part of the oscillation output B of the oscillator 12 is supplied to and amplified by the voltage control amplifier 10 and then applied to the adder 14 as an AC bias signal. The amplified AC bias signal B is added to or superimposed onto the input signal A from the input terminal 13 in the adder 14, and the output of the adder 14 is applied to the magnetic recording head 1.
A part of the output of the adder 14 is supplied to the low-pass filter 4 to obtain a signal in which high frequency components are cut off. The cut-off frequency of the low-pass filter 4 is set to a substantially center frequency of the band of signals to be recorded.
The above-mentioned biasing effect affecting a signal itself (called self-biasing effect) and the biasing effect of a high-frequency band signal affecting a low-frequency band signal (called mutual bias effect) are conspicuous on the high-frequency band side of the signal. Therefore, these effects can be compensated by the low-pass filter 4.
The output of the low-pass filter 4 is supplied to the peak detector 5. The quantity of effective bias detected by the peak detector 5 is supplied to the differential amplifier 6 in which it is compared with the voltage of the reference voltage source 9.
The voltage of the reference voltage source 9 is set to a value corresponding to the optimum bias quantity depending on the kind of a magnetic tape to be used.
The output C of the differential amplifier 6 serves to control the gain of the voltage control amplifier 10 to thereby keep the quantity of effective bias constant. That is, when the quantity of effective bias is insufficient, the output voltage of the peak detector 5 becomes lower than the voltage of the reference voltage source 9 so that the output voltage of the differential amplifier 6 becomes higher. As a result, the gain of the voltage control amplifier 10 becomes larger so that the quantity of effective bias increases.
On the contrary, when the quantity of effective bias is excessive, the output voltage of the peak detector 5, which detects the quantity of effective bias, becomes higher than the voltage of the reference voltage source 9, so that the output voltage of the differential amplifier 6 becomes lower. As a result, the gain of the voltage control amplifier 10 becomes smaller, thereby decreasing the quantity of effective bias.
According to the circuit of FIG. 3, it is possible to eliminate the aforesaid problem that the quantity of effective bias of the recording head is made deeper by the biasing effect affecting a signal itself to be recorded.
When the circuit in FIG. 3 is constituted by a semiconductor integrated circuit in order to be applied to, for example, an audio tape recorder or the like, however, there arise the following problems. That is, when a tape such as a metal tape or the like having a relatively large saturated magnetic field is used as a magnetic tape, a large quantity of bias is required and hence the terminal voltage of the recording head 1 takes a large value.
For example, if the bias frequency is selected to be about 100 kHz, the terminal voltage of the recording head 1 becomes about 100 Vpp. If such a high voltage is applied to the low-pass filter 4, an amount of power consumption is increased therein, and it is therefore necessary to set a value of the resistor 2 not less than 100K.OMEGA. in order to prevent the power consumption from increasing.
Further, if the low-pass filter 4 is constituted by such a resistor 2 having a large value, it is necessary to set the input impedance of the succeeding peak detector 5 to be not less than five times or, generally, to be about ten times as large as the value of the resistor 2 to so that the input impedance does not influence on the cut-off characteristic of the low-pass filter 4.
Since the cut-off frequency of the low-pass filter 4 is selected to be about 10 kHz, the input signal level of the peak detector 5 has a large amplitude of about 10 Vpp. It is therefore necessary to make the allowable input level of the peak detector 5 large enough as well as to secure a dynamic range. That is, the power source voltage must be made large. Further, such a large signal level requires a high-speed rising characteristic.
It has been therefore difficult to realize the circuit of FIG. 3 with a semiconductor integrated circuit.